1. Field of the Invention
The present invention relates to a method of analyzing a semiconductor device fabrication process. More particularly, the present invention is directed to a method of confirming the state of a process step by analyzing the concentration of an impurity in a wafer or in a layer formed thereon, or by analyzing the concentration profile of an ion or element injected into the wafer or a layer thereupon during the fabrication process.
2. Discussion of Related Art
Generally, a semiconductor device is fabricated in such a manner that many processes like photolithography and thin film formation are carried out on a wafer repeatedly. When these processes are performed, the state of the process is confirmed at all times by analyzing the concentration of an impurity in the wafer or in a layer formed on the wafer, or by analyzing the concentration profile of an ion or element injected into the wafer or layer. This analysis is conventionally performed through the process of Secondary Ion Mass Spectrometry(SIMS). As depicted in FIG. 1, a secondary ion mass spectrometer 1 carries out this process. The spectrometer 1 includes a vacuum chamber 2, primary ion generator 3, electrostatic magnetic field 4, mass analyzer 5 and detector 7.
A conventional method of analyzing an impurity concentration using the process of Secondary Ion Mass Spectrometry is explained using FIG. 2. One of the wafers 6 used in the fabrication process is selected. Then a specific portion of the selected wafer 6 is cut to a size of approximately 1 cm.times.1 cm. By doing so, a plurality of samples 6a are prepared. Before such a sample 6a is placed in the vacuum chamber 2 for analysis, it is placed in a subchamber (not shown) and the subchamber is evacuated in order to maintain a vacuum pressure of 10.sup.-6 to 10.sup.-7 torr. Thereafter, the sample 6a is placed in the vacuum chamber 2 of the secondary ion mass spectrometer 1, which always maintains a vacuum pressure of 10.sup.-9 to 10.sup.-10 torr.
Returning to FIG. 1, after the sample 6a is placed in vacuum chamber 2, primary ions are irradiated to the surface of sample 6a so as to generate secondary ions from the wafer or a layer formed on the wafer. For the secondary ions passing through the electrostatic magnetic field 4, only these secondary ions that have energy of a specific level can pass the electrostatic magnetic field 4. Calibration of the energy level and thus, measurement of the secondary ions is accomplished by controlling the intensity of the electrostatic magnetic field 4.
The mass of the specific secondary ions which pass the electrostatic magnetic field 4 is analyzed through mass analyzer 5 and detected by detector 7. By using this technique, it is possible to confirm the existence of a minute amount of an impurity contained in a wafer or a layer formed on the wafer, and to obtain a concentration profile for the depth of the impurity in the substrate or layer.
When the analysis for one sample 6a is completed, the sample 6a located in vacuum chamber 2 is moved to the subchamber(not shown), and the pressure of the subchamber is returned to the normal atmospheric level. Then, another sample 6a is analyzed by repeating the aforementioned procedure.
In conventional analysis methods using a secondary ion mass spectrometer or a similar analyzer, approximately five samples are taken from one wafer 6, and the same analysis procedure is carried out five times. Accordingly, more analysis time is required to handle all five samples, and the procedure is very complicated because the subchamber must be repeatedly pumped in order to maintain its vacuum at a predetermined level, and then the subchamber is discharged every time a sample 6a is replaced.
Since wafer 6 must be cut in order to form the sample 6a, the wafer 6 cannot be used in the production of the semiconductor devices. This results in additional costs due to the loss of production. Moreover, an accurate analysis is difficult to achieve because only one wafer is sampled and analyzed from among a large number of wafers. This is especially true since an accurate impurity profile even in one wafer may not be readily obtained. Accordingly, analysis reliability is poor.